Echo suppressor



Aug. l2, 1930. D. MITCHELLET AL 1,772,550

ECHOv sUPPREssoR Filed Feb. 2o, 1929 gtc//ael @10. Sfilel A ATTORNEY l PatentdAug. 12, 1930 UNITED -s'iti'riais PATENT OFFICE DDBEN IMITCHELL, OINEW YORK, N.` Y., AND HAROLD GfBILENT, 0l' LOS ANGELES, GALII'OBNIA, .LSBIGNOBS TO AMERICAN TELEPHONE `ANTI) TELEGBAIII COMPANY,

a eoaroiwrioie oir Naw Yonx sono surraasson Application led February 20, 1929. Serial No. 841.540.

This invention relates to four-wire transy mission circuits, and. more particularly to means for suppressing echoes in such circuits.

QFor a discussion of some of the funda- Amental problems connected with the presence of echoes in two-way transmission circuits 'and for va description of echo suppressing devices which are among. those better known y in the art, reference may be had to an article 10-by A. B. Clark and It. C. Mathes, Journal f the American Institute of Electrical Engineers, June.1925, pages 618-626.v Ii'i a four-lwirevcircuit, the echo suppressor--which includes twol amplifier-detectors, ,f each having its input connected across one ;o'f the two-Wire transmission paths and each "operating a'relay or a plurality of relays to block the opposite transmission path and to v prevent false operation ofthe opposite ainplifier-detector-niay belocated approximatel 'at the middleof` the four-wirecircuit. In this case, the relays controlling the short` circuiting of lthe transmission paths mustI have a: slow release period equal to the time g5 required for a wave to g'o from the echo suppresser point to either end of the circuit, plus the time required for the echo to return to the suppressor point; accordingly, the trans- A, mission path is blocked for a period of time lconsiderably,greater than that during which the echo is' present in, the'system, especially if the circuit is a lon one, In accordance wit another arrangement, Athe amplifier-detectors andthe relays controlled yvthem are separated, one am lifierdetect'br and its associated relays being located at each end., of the four-wire circuit.

In this case, the hang-over of the relays which shortfcircuit the two-wire aths may be. rev duced to a very4 short period of time, and accordingly. ,the transmission circuit is blocked only while an echo is actuall pres- "en't.. in the s stem. However, with t is ar` "rangement, if the talker' at the west end of 4.5 the circuit, for instance, and the talker at 'the east end of the circuitfbe 'nto s eak at practically the same time, bot ampl fier-de# tectorswill be operated, and as a' result, speech will be completely blocked in both transmission paths. This serious disadvan' begins to speak. The necessarily long and tage is overcome if telegraph circuits are employed, one-in association with each ampli-4 fier-detector to operate a rela' ,at the -distant end for the purpose of disab ing the distant amplifier-detector when the near-end talker extremely fast -operating telegraph circuits are, however, very expensive, and thus the arrangement will, in many cases, b e 'impracn ticable. y

The object of the present invention is to provide an' echo-suppressing arrangement which combines the abovefmentioned advantages without the requirement of fast operating and expensive telegraph circuits. More specically, the invention aims to secure in an economical'fashion the satisfactory su l rlessioii of echoes while avoiding excessive y, ong` blocking of one transmissioii'path and the ikelihoo of complete blocking of speech in'both pathax` 7 I y In general, the applicants, in attaining thev iabovle stated object, provide. a voice-*operated device connected across each of the two-wire paths of the four-wire circuit-ap roximately at the midpoint of the circuit, or instances-means responsive to the o eration of each'voice-operateddevice .for b ocking the opposite transmission path and disablingthe voice-operated device connected across that ,so path, and means associated with the input-of' each v oice-o erated device for delaying'the operation o the devicelover ,the periodo'f time re -uired for a wave totravel'froni'-the point o connection in onetrans'mis'sion path' 85 to the correspondingypoint in the opposite y transmission path. I, c 4

`The invention .'llbe clearl ,understood when the following detailed escription is read with reference tol the' accompanying drawing. .Figure 1 of the drawing shows diagra-mmatically, and in part s cheniaticall one .desirable form of the invention, whi e v Fi 2 shows iii like manner a slightly iiiodi-l He` y f '95 th reference, first, to Fig. 1 of the draw- 1 ing, is shown the.` mid-section of' at four-'wine transmission circuit, the ends" of' this circuit being designated for conveniencev West and East. 'The upper' two-wire M path L is adapted for transmission from west to east and includes the one-way amplifier A. Similarly, the lower two-wire path L1 is adapted for transmission from east to west and includes the one-way amplifier A1. An amplifier-detector AD is connected across the transmission path at a point X through a delay circuit D. A relay R is connected in the output of the amplifier-detector AD and is designed to close a circuit through a battery B and the windings of relays S and SS. The latter relay is slow releasing. The short connecting circuit, which provides for the control of relays S and SS in response to the operation of relay R, is designated C.

1. An amplifier-detector AD1 is' connected.

across the transmission path L1 at a point Y through a delay circuit D1. The amplifierdetector AD1 controls a relay R1 which, in turn, is designed to close a. circuit through a batter B1 and relays S1 and SS1. The relay S 1 is slow releasing.l The short circuit connecting the relay R1 with the relays S1 and SS1 is desi ated C1. It is to be noted that the relay 1, upon operation, places a short-circuit across the branch path from the point X in line L at a point ahead of the delay circuit D, while the relay SS1 is designed to short-circuit the branch path between'the delay circuit D and the input of the amplifier-detector AD. It will be noted further that the functions of these two relays correspond tothe functions of relays S and SS, respectively, on the other side of the echo su ressor circuit.

The de ay introduced by the delay' circuit 4D or by the delay circuit D1 is equal to the period oftime required for a wave to' travel rom the point of connection of the echo suppressor in one transmission path to thecorresponding point in the opposite transmission path. More specificall D introduces a delay of thev time require for a wave to travel from X in path L tothe east end.

of the two-wa circuit and back (as an echo) to the point Y in path L1, while D1 introduces a delay equal to lthe time required for a wave to travel `from Y to the west end of the two-way circuit and back (as an echo) to the point X.

The operation of the circuits of 1 will now be described. v

Let it be assumedthat the talker at the west end of the two-way circuit wishes to take control and that there is no speech coming at the moment from the east end of the.

circuit. The voice currents pass over the two-wire line L through the amplifier A and on to the east end. of the circuit where they may be reflected back over the path L1 as echoes. A part of the currents in L will be drawn 0H at X into the branch path toward the amplifier-detector AD. In response-to the operation of AD, the relay R closes the circuit C and causes the operation 4of relay S and the operation of relay SS. Since relays R and S are quick-operating, a shortto the east end of the four-wire circuit, no

hang-over of the vrela S will berequired, and the path L1 will be blocked only for the time required to cut off the echo. It will be seen hereinafter that the connection to the fourwire circuit of a two-wire circuit, or of a combination of two-way circuits, calls for a short hang-over.

It is, of course, possible that immediately after thev talker at the west end of thc circuit .has started to speak, the talker at the east end may attempt to takecontrol of the circuit. In this case, the voice currents in the line L1 would reach the point Y before the echoes ofthe voice currents originating at the west end and traveling over the line L. The voice energy passed into the branch path at Y may be stored up inthe delay circuit D1 and would eventually cause the operation of the amplifier-detector AD1.

The short-circuit placed on the branch path from Y by lthe relay S disables the ampllfierdetector AD1 and also laces a short-circuit on the input of the de ay circuit D1. This relay S, however, does not operate until after the delay introduced by the circuit D. Accordingly, it becomes necessary to prevent the delayed operation of the ampliier-detector AD1 and the resultant false blocking of the path L by the currents stored up in the circuit D1 as a result of the speech from the east end of the circuit. The prevention of this false operation of amplifier-detector AD1 is the function of relay SS. This relay is given a hangfover long enou h to hold the.

short-circuit on the input o amplifier-detector yAD1 until the delayed currents have passed through the delay circuit D1. This slow release period would, of course, be aplproximatelyequal to the delay introduced by the delay circuit D1. l

The operation of the echo suppressor circuits in response to voice currents from the east end of the two-way circuit is of course lis similar to the o ration responsive to voicel currents in the line L. The amplifier-detector AD1 is operated, but only after the delay introduced by the delay circuit D1. The amplifier-detector operation causes the operation ofrelays R1, S1, and SS1. Relay S1 places a short-circuit on the transmission line L at X 'and also shorts-,the input to the delay eiri of :the fourswire circuit to an end of the twow'aysciuit and back to the echo Suppressor point, may be .2 second or greater. Ifthe transmission path is blocked for a period of ..2 second or longer, it becomes difficult for a talker to break the control from the opposite endpand take control himself. In avery longV four-wire circuit having circuits lconnectedl toits ends of considerable length,y the use of the ordinary type of echo suppressor would' require a hang-over for the two-wire blocking' relays of about .35 second. rIhis long hangj over of course would lcause diiiiculty in breakv ing between the two'talkers. However, Aassuming that the delay time of the fourewire circuit-is .25 second,'and the additional de "lay of .1 second isvrequired by the connected circuits, it is evident that if the. applicants improved echo'suppressor is employed, the Inecessary hang-over' of the relays S and S1 may be reduced from. .35 second to .1 second.

A further advantage of the applicants arrangementis 4foiuid in the fact that since the delay circuits are used only inthel kbranch paths for the control of the echo suppressor l and no't in the main-'transmission paths, a

relatively inexpensive type of mechanical or accoustic delay circuit may be employed in place of the delaycircuits'often'used in the transmission paths. j

-The arrangements of echo-suppressor cir cuits shown in Fig. 2 of the drawing are somewhat similar to those shown in Fig. 'l

but diiler from the circuits of Fig. l in that the functions of actually suppressing echoes in the .transmission path and preventing false operation ofthe opposite echo suppressor devices areseparated. The various elements of Fig. 2 are designated by the same refer ence letters as the corresponding elements of Fig. 1 except that the letters `of Fig. 2 are primed. It isbelievedthat the circuits of Y Fig. 2 will be understood with sufficient clartheir operation.

ity from the following brief description of If the talker at the Iwest end is the tra a) speak, the amplifier-detector. AD' is operated after the delay introduced by the delay circuitD. Relay R- closes .the` circuit C through thel battery `B and relays S and SS. Relay S places a short-circuit on the February, 1929. line L1L at a point near the mid-point of the four-wire circuit, for instance, `and relay SS', which is slow-releasing, prevents `the operation of amplifier-detector ADJL and the echo suppressor relays controllingthe line L.

Similarl ,j voice currents in the line Ll.

SS and SS1 should each have a desira. le, it is'to bel understood that its true scope 1s Ynot limited thereby but is to be de` terminedfrom the appended claims. Y

What is claimed is:

1. *In a fourfw'ire circuit having a first path adapted for transmission in one direction and a second path 4'adapted for transmission in the opposite direction, la voice-operated device connected across each of said paths, means responsive to the Voperation of each 'of Vsaid voice-operated devices for blocking the opposite transmission path and disabling the vvoice-operated .device connected across' said opposite path, and means associated With each of said voice-operated -devices for VdeIayiIigL-the operation thereof over a periodvof` time not greater than that required for a wave to, travel fronnthe point of connection in one transmission path to the corresponding point in the opposite transmission path.

2.' In a four-wire circuit having a first path adapted fortransmi'ssion in onedirection and a second-path ada ted for-transmission in the opposlte direction, a voice operated device connected across each Aof said paths, means responsive'to the operation of each of said voice-operated devices yfor blocking the opposite transmission path and disabling the voicefoperated device connected across said opposite path, means associatedwith each of said voice-operated devices for delaying the operation thereof over a period of time not greater than that required for a wave to travel from, the point of connection in one transmission pathl to the corresponding point inthe opposite transmission path, and means associated with each of said voice-operated devices for holding the device in its disabled condition to prevent'false operation thereof.

1 DOREN MITCHELL.

' In testimony whereof, I have signed' myy name to this specification this 13th day of HAROLD o. siLENr.

so has been disclosed 1n 

